Analysis of damage and fracture mechanisms in ductile metals under non-proportional loading paths

The paper discusses biaxial experiments and corresponding numerical simulations to analyze the effect of non-proportional loading paths on damage and fracture behavior of ductile metals. Newly developed specimens are taken from thin metal sheets and are tested under different biaxial loading conditions covering a wide range of stress states. In this context, an anisotropic continuum damage model is presented based on yield and damage conditions as well as on evolution laws for plastic and damage strain rates. Different branches of the damage criteria are taken into account corresponding to various damage and failure processes on the micro-level depending on stress triaxiality and Lode parameter. Experiments with biaxially loaded specimens have been performed. Results for proportional and corresponding non-proportional loading histories are discussed. During the experiments strain fields in critical regions of the specimens are analyzed by digital image correlation (DIC) technique while the fracture surfaces are examined by scanning electron microscopy (SEM). Numerical simulations of the experiments have been performed and numerical results are compared with experimental data. In addition, based on the numerical analyses stress distributions in critical parts of specimens are detected. The results demonstrate the efficiency of the new specimen’s geometries covering a wide range of stress states in the shear/tension and shear/compression regime as well as the effect of loading history on damage and fracture behavior in ductile metal sheets

Modeling of ductile damage using numerical analyses on the micro-scale

The presentation deals with a continuum damage model which has been generalized to take into account the effect of stress state on damage criteria as well as on evolution equations of damage strains. It is based on the introduction of damaged and corresponding undamaged configurations. Plastic behavior is modeled by a yield criterion and a flow rule formulated in the effective stress space (undamaged configurations). In a similar way, damage behavior is governed by a damage criterion and a damage rule considering the damaged configurations. Different branches of the damage criterion are considered corresponding to various damage mechanisms depending on stress intensity, stress triaxiality and the Lode parameter. Experiments with carefully designed specimens are performed and the test results are used to identify basic material parameters. However, it is not possible to determine all parameters based on these tension and shear tests. To be able to get more insight in the complex damage behavior under different loading conditions, additional series of micro-mechanical numerical analyses of void containing unit cells have been performed. These finite element calculations on the micro-level cover a wide range of stress triaxialities and Lode parameters in the tension, shear and compression domain. The numerical results are used to show general trends, to develop equations for the stress-statedependent damage criteria, to propose evolution equations of damage strains, and to identify parameters of the continuum model

New approach for determination of strain rate sensitivity of mild steel dc01 under stack compression and uniaxial tensile test

Deformation under uniaxial tensile loading with using Digital Image Correlations (DIC) is the easiest way to analyze the material behavior in sheet metal forming. In order to determine the plastic parameters such as hardening, anisotropy and strain rate sensitivity at higher strain level, equi-biaxial stress state is prerequisite. As reported in the literature, Bulge tests are frequently used for this purpose, but in this work, stack compression test is used as an alternative. In this experiment, deformation in the middle layer where the friction effect is the lowest was monitored using two pairs of DIC systems in rolling and transversal directions. Uniaxial tensile tests as well as stack compression tests were performed on mild ferritic steel DC01 at different strain rates, from 0.001 −1 to 10 −1. Strain rate sensitivity parameter was investigated at different level of strains for both experiments and strain rate sensitivity profiles were obtained. Results show a decrease of material strain rate sensitivity with increasing the true strain

New 2D-Experiments and Numerical Simulations on Stress-state-dependence of Ductile Damage and Failure

AbstractThe paper deals with a series of new experiments and corresponding numerical simulations to be able to study the effect of stress state on damage and failure behavior of ductile metals. The material behavior is modeled by a continuum approach based on free energy functions defined in damaged and corresponding fictitious undamaged configurations leading to elastic material laws which are affected by damage. Inelastic behavior of ductile materials is modeled by continuum plasticity and continuum damage model, respectively. The present approach takes into account the effect of stress state on damage and failure conditions expressed in terms of the stress intensity, the stress triaxiality and the Lode parameter. Previous studies have shown that it will not be possible to propose the stress-state-dependent functions for damage and failure criteria only based on tests with uniaxially loaded specimens. Therefore, new experiments with carefully designed and two-dimensionally loaded specimens have been developed. Corresponding numerical simulations of these tests show that they cover a wide range of stress states allowing validation of stress-state-dependent functions for the damage criterion and evolution laws for the damage strains

Studying Li dynamics in a gas-phase synthesized amorphous oxide by NMR and impedance spectroscopy

Li diffusion parameters of a structurally disordered Li-Al-Si-oxide prepared by gas-phase synthesis were complementarily investigated by both time-domain NMR techniques and impedance spectroscopy. The first include 7Li NMR spin-lattice relaxation (SLR) measurements in the laboratory as well as in the rotating frame of reference. An analysis of variable-temperature NMR line widths point to an activation energy E a of approximately 0.6 eV. The value is confirmed by rotating-frame SLR NMR data recorded at approximately 11 kHz. Above room temperature the low-temperature flank of a diffusion-induced rate peak shows up which can be approximated by an Arrhenius law yielding E a = 0.56(1) eV. This is in very good agreement with the result obtained from 7Li spin-alignment echo (SAE) NMR being sensitive to even slower Li dynamics. For comparison, dc-conductivity measurements, probing long-range motions, yield E a = 0.8 eV. Interestingly, lowtemperature SAE NMR decay rates point to localized Li motions being characterized with a very small activation energy of only 0.09 eV. © by Oldenbourg Wissenschaftsverlag, München

Ultrafast proton-coupled isomerization in the phototransformation of phytochrome

The biological function of phytochromes is triggered by an ultrafast photoisomerization of the tetrapyrrole chromophore biliverdin between two rings denoted C and D. The mechanism by which this process induces extended structural changes of the protein is unclear. Here we report ultrafast proton-coupled photoisomerization upon excitation of the parent state (Pfr) of bacteriophytochrome Agp2. Transient deprotonation of the chromophore’s pyrrole ring D or ring C into a hydrogen-bonded water cluster, revealed by a broad continuum infrared band, is triggered by electronic excitation, coherent oscillations and the sudden electric-field change in the excited state. Subsequently, a dominant fraction of the excited population relaxes back to the Pfr state, while ~35% follows the forward reaction to the photoproduct. A combination of quantum mechanics/molecular mechanics calculations and ultrafast visible and infrared spectroscopies demonstrates how proton-coupled dynamics in the excited state of Pfr leads to a restructured hydrogen-bond environment of early Lumi-F, which is interpreted as a trigger for downstream protein structural changes

Dry-jet wet spinning of thermally stable lignin-textile grade polyacrylonitrile fibers regenerated from chloride-based ionic liquids compounds

In this paper, we report on the use of amorphous lignin, a waste by-product of the paper industry, for the production of high performance carbon fibers (CF) as precursor with improved thermal stability and thermo-mechanical properties. The precursor was prepared by blending of lignin with polyacrylonitrile (PAN), which was previously dissolved in an ionic liquid. The fibers thus produced offered very high thermal stability as compared with the fiber consisting of pure PAN. The molecular compatibility, miscibility, and thermal stability of the system were studied by means of shear rheological measurements. The achieved mechanical properties were found to be related to the temperature-dependent relaxation time (consistence parameter) of the spinning dope and the diffusion kinetics of the ionic liquids from the fibers into the coagulation bath. Furthermore, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical tests (DMA) were utilized to understand in-depth the thermal and the stabilization kinetics of the developed fibers and the impact of lignin on the stabilization process of the fibers. Low molecular weight lignin increased the thermally induced physical shrinkage, suggesting disturbing effects on the semi-crystalline domains of the PAN matrix, and suppressed the chemically induced shrinkage of the fibers. The knowledge gained throughout the present paper allows summarizing a novel avenue to develop lignin-based CF designed with adjusted thermal stability